In gas chromatography (GC) the sample to be analysed is introduced into the inlet end of a separation column and carried through the column by a carrier gas which is necessarily of high purity. To retain the sample to be analysed in the gas phase, it is usually necessary to heat the column to an elevated temperature. The inlet end of the column is usually above atmospheric pressure to allow the gas to be forced through the column. Consequently, the connection at the inlet end of the column needs to be gastight so as not to allow leakage from the column or introduce contamination into the column. The outlet end of the column can be connected to a number of devices, usually a detection device of some sort, and can also be the inlet of another column. The outlet end of the column may be at, below or above atmospheric pressure. The connection at the outlet end of the column will also be gastight to avoid introduction of contamination or leakage.
To seal the column at both inlet and outlet ends, conventional fittings employ ferrules that deform onto the outside of the column to make a substantially leaktight seal with the column as well as with the fitting, thereby forming a seal against the outside atmosphere.
Historically, when capillary columns were metal tubes, various types of ferrules were used including metal ferrules. Glass columns were introduced in the 1960s and immediately the all metal connection systems were unsuitable since they promptly broke the column. This was resolved by employing softer materials, notably silicon rubber, Teflon, graphite and, the most common, a Vespel (graphite polyimide) composite. The essential functional principle of the connection remained as before, but the seal was achieved without breaking the glass column.
Both graphite and the various types of polyimide ferrules had recognised deficiencies. Graphite ferrules were permeable to oxygen, which causes problems with detectors and damage to capillary columns. Furthermore graphite is an extremely absorptive material and will absorb sample constituents that are being analysed in the chromatographic system that it comes in contact with. The ferrule should not normally come in contact with sample components but it is recognised as a problem that pieces of graphite can become detached or extrude into parts of the chromatographic system thus destroying the integrity of the chromatographic separation. Whenever a capillary column was inserted through a graphite or polyimide ferrule to make a connection, a section of the column had to be cut off in case there were particles of ferrule material around the end of the column.
One of the main problems with polyimide and graphite-impregnated polyimide ferrules was that after an initial leaktight seal was made and the chromatographic system was heated, as is required in chromatographic analysis, and then the system cooled down in preparation for the next analysis, the polyimide ferrule developed substantial leaks necessitating the retightening of the fitting to re-establish the leaktight seal. On subsequent temperature cycles a leak might or might not develop. The thermal coefficients of expansion of polyimide materials were significantly higher than those of the metal components encapsulating them. This put the ferrule under significant compression at high temperatures causing it to creep dimensionally. As the fitting system was cooled, the polyimide ferrule contracted more than the metal fitting encapsulating it, which caused gaps and leakage paths to be formed.
In 1979, fused silica tubing was introduced. Fused silica is a high purity form of glass, which made it attractive for high accuracy, sensitive analysis. In addition this tubing could be produced with a much thinner wall thickness that allowed the tubing to be flexible in the same way as a fiber optic. This material is only strong and flexible without breaking when it is kept in the pristine state. Any damage from scratches (even from dust particles) or chemically from moisture in the environment causes the material to be highly brittle and structurally unstable so that it easily fractures. Like fiber optics, this issue was addressed by supplying the fused silica tubing with a fine protective coating that in the vast majority of products was a polyimide coating of thickness typically in the range 5 to 100 micron.
The connection systems used for these gas chromatography fused silica columns continued to be the systems carried over from the glass columns that preceded the fused silica columns, that is systems containing ferrules made of graphite or graphite polyimide composite. The aforementioned problems arising from reliance on such ferrules remained and were reluctantly tolerated and compensated for, accepted as the “downside” price of employing columns that were themselves a very satisfactory vehicle for the gas chromatography technique.
For many years, these various deficiencies of existing graphite and polyimide ferrule systems diminished the reliability of chromatography systems, leading to compromised performance as well as non-productive time in rectifying the faults. In addition, damage to components in the systems regularly occurred when operators applied excessive force to fittings in efforts to eliminate leaks.
International patent publication WO 01/73338 disclosed the concept that, in contrast to longstanding wisdom, metal ferrules could be employed to seal connections to fused silica capillary columns because it was possible to make an effective seal at the interface between the metal ferrule and the polyimide coating on the column without fracturing the underlying glass. It was realised that the coating, fine as it was, made the use of metal ferrules feasible.
The structure disclosed in WO 01/73338, and the actual product brought to market by the assignee, did continue to share one disadvantage with the predecessor arrangements that relied on graphite or polyimide ferrules: in gas chromatography, a tool such as a wrench or spanner lever was necessary to reliably deliver the necessary compression force. The spanner tightening approach has also been accepted as part of what is required to connect thin wall fused silica tubing into a gas chromatography system despite the problems caused by practitioners overtightening fittings, including damage to threads and separation columns and breaking of expensive fittings in the system.
In contrast, fitting systems used in liquid chromatography (LC) can be tightened with just the application of finger force but these have very different requirements in terms of the level of leak tightness and high temperature needs, and they are not sealing directly onto thin wall fracturable glass. Liquid chromatography systems rely on polymer materials like PEEK which has a low enough modulus that sufficient force can be generated to adequately deform the ferrule without the leverage of a spanner. These fingertight fittings are the standard approach in LC. The reason that these have never been used in gas chromatography is the requirement for high temperature operation and sealing of very non-viscous and potentially hazardous gases like hydrogen. There have been a number of attempts over the last 20 years to commercialise simplified GC connector systems that did not require spanners but none have been successful.
An object of the invention is a ferrule and fitting design that allows fused silica tubing to be sealed into a gas chromatography system by finger force alone, i.e. without recourse to spanners or like tools and using the force applied by the fingers (including the thumb) alone without use of leverage or force from the wrist, elbow or shoulder.